1. Technical Field
The invention relates to compacting or consolidation of a bundle of nuclear fuel elements into a tight array so as to reduce the cross dimension of the bundle with a view to storing same.
2. Prior Art
Spent fuel assemblies removed from a reactor must be stored in a pool until they have been sufficiently deactivated before the fuel elements are reprocessed. It is desirable to reduce the volume occupied by the bundle of fuel elements of an assembly as much as possible so as to increase the storage capacity of the pool. It has been proposed to separate the skeleton from the fuel elements, which form the most radioactive elements, and to form these latter into a compact bundle in which the elements are tightly pressed against each other and distributed in a triangular network.
In most fuel assemblies, particularly in light water reactors, the elements of a fuel assembly are distributed in a square lattice with some fuel elements substituted with elements of a different kind, such as guide tubes belonging to the skeleton of the assembly. It is necessary to remotely extract the fuel elements from the assembly, under water due to their residual activity and due to the release of heat, and to form them into a tight bundle, and then to insert it into a storage case.
Numerous devices have already been proposed for reaching this result and some of them have been constructed. Most of the devices use the fact that, due to their great length and their small diameter, the fuel elements have a sufficient flexibility to bend when passing from one pattern to another. French No. 2,528,218 describes a method which consists in sliding the elements into a compacting device formed of guide tubes located for array transformation and extended by passages defined by longitudinal rods. The reliability of such a system for compacting fuel elements which have been subjected to irradiation may be challenged. In addition, the absence of elements at some locations in the original lattice, where they are replaced by tubes belonging to the skeleton, results in empty spaces in the tight array and discontinuities of support which may cause elements to overlap.
French No. 2,506,993 (U.S. Pat. No. 4,446,098) describes an apparatus comprising several devices brought into play successively and including a compacting device. This latter comprises a vertical quiver in which the elements of a bundle are all inserted at the same time from the top, in successive frames which force them first of all into a rectangular mesh mattice, then into a square lattice with tight spacing, finally into a triangular lattice. The device is made from three sections of approximately pyramidal shape. It is complex and requires forcing the elements to be inserted by means of rollers so as to overcome the resilient resistance to bending offered by the elements.
It is an object of the invention to provide a compacting method and an apparatus which do not subject the elements to flexions which would require high insertion forces; it is a further object to form a "truss" in which the elements are distributed without leaving any empty space in a tight triangular lattice.
To this end, there is provided a method for forming a bundle of fuel elements disposed in a square mesh arrangement, extracted from a fuel assembly, into a tightly packed fuel element bundle with a triangular mesh arrangement, wherein:
(a) all fuel elements are taken simultaneously from the assembly, except those which are placed in lines adjacent those which comprise unoccupied locations of the square lattice, and which are not adjacent an unoccupied location;
(b) the elements removed are lowered into a vertical quiver of pyramidal shape having surfaces converging towards the bottom and the two opposite faces of which have projections for placing the elements in a quincunx arrangement,
(c) a plurality of groups of layers of elements are spaced symmetrically apart from each other so as to create free reception spaces, at the top part of the quiver,
(d) all the remaining elements are grasped, turned as a whole through 90.degree. and lowered into the free reception spaces between groups of layers (the grasping head having been previously rotated through 90.degree. so as to grasp the remaining elements),
(e) and the layers of elements are moved toward each other by moving the quiver along the elements so as to form a compact truss.
The invention also provides a compacting device for implementing the above-defined method comprising: a vertical quiver of pyramidal shape having lateral faces converging towards the bottom and of rectangular cross-section, two opposite faces of the quiver being provided with projections whose height is greater near the bottom so as to change the configuration of fuel elements from a square to a triangular pattern; dividing walls perpendicular to said faces provided with projections, for separating and guiding lines of elements in an upper part of the quiver, defining compartments for introducing elements and formed in several fractions which may be moved apart and toward each other in a direction parallel to the dividing walls.
The invention will be better understood from the following description of embodiments given by way of examples only.